1. Field of the Invention
The present invention relates to a color wheel and a projector which incorporates the color wheel.
2. Description of the Related Art
In these days, projectors are used in many cases as an image projection apparatus for projecting screens and video images of a personal computer and, further, images based on image data stored in a memory card on to a screen.
According to the configuration of these projectors, a light source unit including a small high-luminance metal halide lamp or a high-pressure mercury lamp is incorporated and light emitted from the light source is passed through a color wheel of a color wheel unit for coloring. The light so colored is then passed through a light guide rod or light smoothing rod and is, thereafter, concentrated on to digital micromirror devices referred to as DMDs via lenses and mirrors, so that a color image is displayed on to a screen by the amount of light reflected by the micromirrors in the on position towards a projection opening of the projector via a projection lens.
The color wheel unit of the projectors is made up of a color wheel having red, green and blue filters and a wheel motor for rotating the color wheel at a rotational speed of about 120 revolutions per second.
Incidentally, this color wheel is made up of a plurality of filter pieces and is generally formed into a three-segment color wheel including segments of red, green and blue, which are “primary colors”. However, in order to prevent color separation by a rainbow effect, there sometimes occur cases where six-segment three-color color wheel is used which includes red, blue, green, red, blue and green segments.
In addition, in order to realize an increase in color reproduction characteristics by increasing the luminance, there are cases from time to time where a color wheel is used which has one or more segments added to the three-segment color wheels. This color wheel having four or more segments includes a colorless filter piece and/or filter pieces for magenta, yellow and cyan which configure complementary colors.
In addition, for example, Japanese Unexamined Patent Publication No. 2007-52059 proposes a projector which includes a color time division unit for dividing white light into light beams of three primary colors, red, green and blue, in a time division manner, DMDs for sequentially modulating the light beams of three primary colors which have been so divided in the time division manner by the color time division unit, and a projection lens for projecting the light beams which have been modulated by the DMDs in combination with one another. In this projector, as the color time division unit, there are provided a color wheel in which filter pieces of magenta, yellow and cyan, which configure complementary colors, are disposed in a circumferential direction in such a manner as to divide the color wheel into three equal segments and a reflection mirror for reflecting the light beams which result when the white light from a white light source passes through the two filter pieces of the color wheel in such a way as to be returned to the color wheel for re-passage. In addition, the reflection mirror is provided in such a manner that two light incident points, an incident point of light from the white light source and an incident point of light from the reflection mirror, lie in an axial symmetry with respect to a rotational axis of the color wheel.
In the related art techniques, however, in the event that the number of colors of a color wheel is increased, since it is general practice to apply a coating which transmits only light of certain wavelengths to each of filter pieces of the color wheel, there has been caused a problem that the number of components is increased. In addition, since a color wheel unit is formed by disposing the respective filter pieces about a rotational shaft of a wheel motor in such a way as to form a disk and then, fixing the filter pieces so disposed to the rotational shaft of the wheel motor with an adhesive or different fixing means, in the event that the number of filter pieces is increased, the position of the respective filter pieces becomes complex, and this causes a problem that an increase in the number of manhours and hence an increase in production costs are called for. Further, there is also caused a problem that it becomes difficult to fix the filter pieces so strong and rigid to the rotational shaft of the wheel motor as to withstand high-speed rotations from the viewpoint of strength.
In addition, in the invention proposed in the aforesaid patent document, the color wheel is provided in which the filter pieces which transmit the light beams of complementary colors to the three primary colors of light are arranged in the circumferential direction, and the reflection mirror is disposed which returns the light beam that has passed through the one filter piece in such a manner as to cause the light beam to pass through the different filter again. By adopting this configuration, the dividing speed of color in the time division manner can be made twice faster without increasing the rotational speed of the color wheel and the number of filter pieces and the color separation due to the rainbow effect can be prevented. However, there needs to provide the reflection mirror for reflecting the light beam that has passed through the filter piece, and this has called for a problem that the configuration in which the optical system including the reflection mirror is arranged becomes complex. In addition, since the light beams of magenta, yellow and cyan, which configure the complementary colors, cannot be used as they are when they have passed through the respective filter pieces, there has been caused a problem that the color reproduction characteristics cannot be increased by increasing the luminance.